This invention relates to improvements in lifting chain assemblies for multistage telescopic lift truck masts. More particularly the invention relates to improvements in "hidden" chain assemblies to facilitate removal, for servicing and replacement, of the lifting chains without requiring disassembly of the mast sections, and to related interference stop structures for preventing hyperextension of the mast sections with respect to one another.
As the usage of highly extensible lift truck masts, particularly of the three and four-stage type, has increased in the materials handling industry, and as the emphasis on improved visibility and maximum load-carrying capacity of the lift trucks has simultaneously increased, various design features have been sought for improving the compactness of such masts. One such design feature has been the location of lifting chains, particularly those used in the telescopic extension of mast sections, within the elongate enclosures normally existing between adjacent mast upright members bounded longitudinally by the transversely-extending longitudinal strengthening flanges typical of such upright members having channel-shaped and I-beam-shaped cross sections. For example, a three-stage mast has previously been devised wherein each of a pair of lifting chains is located within a respective enclosure between adjacent upright members of the outer (i.e. fixed) and intermediate mast sections. Each chain is anchored to the outer upright member by a chain anchor located within the enclosure adjacent the top thereof from which the chain depends downwardly through the enclosure, being trained beneath a sheave rotatably mounted upon the intermediate upright member also within the enclosure. From the sheave the chain extends upwardly and is connected by a second chain anchor to the bottom of a respective inner upright member. In operation, extension of the inner mast section relative to the intermediate mast section exerts a lifting force through each chain on the respective sheaves and thereby simultaneously extends the intermediate mast section with respect to the outer mast section. It will be appreciated that such hidden chain assemblies are very useful in facilitating compact design of the mast since such chains occupy interior spaces which are already present, rather than requiring additional space exterior of the mast sections which would increase the bulk of the overall mast assembly.
A salient problem of such hidden lift chains is their inaccessibility for servicing or replacement due to the position of each chain and at least one of its chain anchors within the aforementioned enclosure. Even access to the interior of the enclosure through the top or bottom thereof is normally obstructed by mast guide rollers. Modern mast design requires the use of such mast guide rollers located within the enclosures between adjacent upright members for engaging the transversely-extending flanges thereof and resisting bending moments while facilitating relative longitudinal movement between the upright members. Normally a pair of cooperating guide rollers are mounted between adjacent upright members, one being fixed to the outer, or lower, upright member adjacent the top thereof and the other being fixed to the inner, or higher, upright member adjacent the bottom thereof. The presence of these guide rollers dictates that those hidden chain anchors positioned within the enclosures between the adjacent upright members be separated from the ends of the enclosures by one of the mast guide rollers. Thus the chain anchor is totally inaccessible and, in order to disconnect the chain therefrom, it has been necessary to disassemble the mast sections from one another so as to gain access to the enclosure to detach the chain. This necessitates a large amount of labor and down time for the lift truck, adding to the cost of its maintenance and reducing its productivity.
A further problem with masts having such hidden chain assemblies is the inability to provide effective interference stops in the enclosure occupied by the hidden chain for preventing inadvertent hyperextension of one mast section with respect to the other. Such stop, to be effective, can be installed only after the two mast sections have been assembled with respect to one another, and must be removable prior to disassembly of the mast sections. The enclosed position of such a stop, and the presence of the guide rollers and hidden chain assembly, would prevent access to permit such installation or removal.